Nose form was shaped by climate

UNIVERSITY PARK — Big, small, broad or narrow, humans inherit their nose shape from their parents, but ultimately, the shape of someone's nose was formed by a long process of adaptation to our local climate, according to a team of researchers.

"We are interested in recent human evolution and what explains the evident variation in things like skin color, hair color and the face itself," said Mark Shriver, professor of anthropology, Penn State. "We focused on nose traits that differ across populations and looked at geographical variation with respect to temperature and humidity."

They considered a variety of nose measurements, looking at the width of the nostrils, the distance between nostrils, the height of the nose, nose ridge length, nose protrusion, external area of the nose and the area of the nostrils. The measurements were made using 3D facial imaging.

Differences in the human nose may have accumulated among populations through time as a result of a random process called genetic drift. However, divergent selection — variation in natural selection across populations — may also be the reason that different populations have differing noses. Teasing the two apart is difficult, especially in humans.

Researchers found the width of nostrils and base of nose measurements differed across populations more than could be accounted for by genetic drift, indicating a role for natural selection in the evolution of nose shape in humans. To show that the local climate contributed to this difference, the researchers looked at the spatial distribution of these traits and correlated them with local temperatures and humidity. They showed that the width of the nostrils is strongly correlated with temperature and absolute humidity

The researchers noted "...wider noses are more common in warm-humid climates, while narrower noses are more common in cold-dry climates."

"It all goes back to Thompson's Rule (Arthur Thompson)," said Shriver. "In the late 1800s he said that long and thin noses occurred in dry, cold areas, while short and wide noses occurred in hot, humid areas. Many people have tested the question with measurements of the skull, but no one had done measurements on live people."

One purpose of the nose is to condition inhaled air so that it is warm and moist. People with narrower nostrils probably fared better and had more offspring than people with wider nostrils, in colder climates. This lead to a gradual decrease in nose width in populations living far away from the equator.

Shriver notes that this is not the only explanation for nose-shape variation in humans. He thinks another way that the cross-population differences in nose size may occur is through sexual selection. People may choose mates simply because they find a smaller or larger nose more attractive. If an entire group thinks small is better, then those with large noses will have less success in reproducing and fewer large-nosed people will be in the group. Over time, the nose size in the group will shrink relative to other groups where large noses are favored. These notions of beauty may be linked to how well-adapted the nose is to the local climate.